This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objective: To assess whether prolactin and oxytocin, commonly thought to be important in parent-offspring relationships is also important in adult relationships between mates. Prolactin is necessary for successful nursing in mammals, and also regulates maternal behavior. In some biparental species of birds and mammals prolactin levels are elevated in fathers, and there is some evidence that prolactin may be important for quality male care of infants. Prolactin, thus, is often thought of as a "parental" hormone. Oxytocin is also involved in maternal care but has recently been implicated in pair bonding in monogamous rodents. Are there other conditions where elevated prolactin and oxytocin might be important? These "parenting" hormones might be important in other social relationships. We have found significant positive correlations between prolactin and oxytocin levels and affiliative behavior (huddle, groom), and sexual behavior in both male and female cotton-top tamarins (Saguinus oedipus). Mates that engaged in more positive social interactions with their mates had higher hormone levels. We have replicated data on variation in affiliative relationships in 15 mated pairs where we simultaneously recorded oxytocin levels. We found a five-fold variation in the amount of affiliative behavior shown, and a five-fold variation in baseline oxytocin levels. Both males and females showed equal levels of oxytocin and levels within pairs were highly correlated and oxytocin levels correlated positively with affiliative behavior. Variance in male oxytocin levels was best explained by variance in sexual activity whereas variance in female oxytocin levels was best explained by variation in contact and touching. The high within-species variation in affiliation and hormone level, the fact that both sexes have equal levels of oxytocin correlated with affiliation and the different behavioral sources of variation for each sex are each novel and exciting. This work relied on the WNPRC Assay Services and Library and Information Services.